I assume this is a reference to group delay. If so, I tend to agree, but believe its worth noting that even if I have a system with group dlay, I would not want to add irregularities on top of that.
You might get me to agree so far as adding diff sized subs is likely to make your system louder over all, but those more specific desired improvements (small subs for upper bass, big subs for lower bass) is almost never real.
Most people try to justify multiple sized subs with some personal theory on frequency response, wave interaction, or some other guess. In the end it always boils down to they think it sounds better that way (ie: cotjones) because they haven't gained the experience to realize that most times, simpler is better, and sounds better. If you want to get stupid loud, sure knock yourself out, run diff subs. But if you think you are doing it for some sonic improvement, think again.
You can get cancellation at 1/4 and 1/2 waveforms btw, easily attainable inside a vehicle. If we couldn't get cancellation of subbass inside a car, changing its position/placement would have no audible affect. We know that to be false, it can make large differences in sound.
Cheers.